The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile, or the like, which employs an electrophotographic method to form an image. In particular, it relates to such an image forming apparatus that is provided with a charging member which charges an image bearing member by making contact with the image bearing member.
Conventionally, a corona based charging device has been used as a means for charging an image bearing member, such as an electrophotographic photosensitive member, an electrostatic recordable dielectric member, or the like, of an image forming apparatus, for example, a copying machine, a printer, and the like, which employs an electrophotographic, electrostatic, or the like recording method . When charging an image bearing member, a corona based charging device is placed close enough to the image bearing member to expose the peripheral surface of the image bearing member to the corona discharge from the corona based charging device so that the peripheral surface of the image bearing member is charged to the predetermined polarity and potential level.
In recent years, a contact type charging apparatus has been realized as a means for charging an image bearing member, because a contact type charging apparatus is advantageous over a corona based charging device in that a contact type charging apparatus produces a smaller amount of ozone, and consumes a smaller amount of electrical power, compared to a corona based charging device. When charging an image bearing member, a contact type charging apparatus, to which voltage is being applied, is placed in contact with the image bearing member, as an object to be charged, so that the peripheral surface of the image bearing member is charged to the predetermined polarity and potential level.
Among various contact type charging apparatuses, a roller based contact type charging apparatus, which employs an electrically conductive roller (hereinafter, xe2x80x9ccharge rollerxe2x80x9d) as a charging member, is preferably employed from the standpoint of charge stability. When charging an image bearing member with the use of a roller based contact type charging apparatus, the charge roller as a charging member is directly pressed upon the image bearing member, while applying voltage to the charge roller, so that the image bearing member is charged.
Referring to FIG. 19, a charge roller 101, that is, the charging member of a conventional contact type charging apparatus, is kept directly pressed upon the peripheral surface of an electrophotographic photosensitive member 100 (hereinafter, xe2x80x9cphotosensitive drumxe2x80x9d) in the form of a rotational drum, as an image bearing member, maintaining a predetermined amount of contact pressure between the charge roller 101 and photosensitive drum 100. As a predetermined voltage is applied to the charge roller 101 from a charge bias power source 102, the peripheral surface of the photosensitive drum 100 is charged to a predetermined potential level. Generally, the charge roller 101 is rotated by the rotation of the photosensitive drum 100 as the photosensitive drum 100 is rotationally driven.
When the photosensitive drum 100 is charged by the charge roller 101 placed in contact with the photosensitive drum 100, the photosensitive drum 100 is charged by electrical discharge. Therefore, the photosensitive drum 100 begins to be charged as a voltage, the potential level of which is higher than a certain level, or the threshold level, is applied to the charge roller 101 from the charge bias power source 102. For example, in order to charge the photosensitive drum 100 having a 25 xcexcm thick photosensitive layer of OPC by directly pressing the charge roller 101 upon the photosensitive drum 100, a voltage of approximately 600 V must be applied to the charge roller 101. In other words, as the potential level of the voltage applied to the charge roller 101 is increased to approximately 600 V, the surface potential level of the photosensitive drum 100 begins to rise, and then, as the potential level of the voltage applied to the charge roller 101 is further increased, the surface potential level of the photosensitive drum 100 linearly increases in proportion to the increase in the level of the applied voltage.
Hereinafter, this threshold voltage value, that is, the value of the DC voltage at which an object to be charged (photosensitive drum) begins to be charged as the value of the DC voltage being applied to the charging member (charge roller) is increased, will be referred to as the xe2x80x9ccharge (discharge) start voltage value Vthxe2x80x9d of the object.
There are different contact type charging methods: xe2x80x9cDC based charging methodxe2x80x9d and xe2x80x9cAC based charging methodxe2x80x9d, which will be described below. In a DC based charging method, only DC voltage is applied to a charging member to charge an object, whereas in an AC based charging method, such AC voltage (oscillating voltage: voltage, the value of which periodically changes with the elapsing of time) that has an AC component and a DC component is applied to a charging member to charge an object.
(1) DC Based Charging Method
When charging an object to be charged (photosensitive drum) to a predetermined surface potential level Vd using a DC based charging method, a DC voltage, the potential level of which is equal to the total of the potential level Vd and the charge start voltage value Vth of the object to be charged (photosensitive drum), is applied to a charging member. A DC based charging method is not satisfactory in terms of charge uniformity. Further, it lacks convergency regarding the potential level higher than Vd. Therefore, an object to be charged (photosensitive drum) needs to be pre-exposed.
(2) AC Based Charging Method
An AC based charging method is superior to a DC based charging method in terms of charge uniformity. When charging an object to a predetermined surface potential level of Vd using an AC based charging method, an oscillating voltage created by combining an offset DC voltage, the potential level of which is equivalent to a predetermined surface potential level to which the object is to be charged, with an AC voltage, the peak-to-peak voltage of which is no less than 2xc3x97Vth, is applied to a charge roller.
This charging method is employed to take advantage of the leveling effect of the AC component represented by the line a in FIG. 20, so that the surface potential level of the photosensitive drum, as an object to be charged, converges to the middle of the top and bottom peak voltage levels of the AC component, as represented by the line b in FIG. 20, according to the gaps between the charging member (charge roller) and the photosensitive drum.
In an image forming apparatus, the charging apparatus for charging the image bearing member (photosensitive drum) uniformly charges the peripheral surface of the image bearing member to a predetermined potential level to form an electrostatic latent image on the peripheral surface of the image bearing member. After the formation of the electrostatic latent image, the charging apparatus removes the electrical charge on the peripheral surface of the image bearing member in order to erase the potential level history on the image bearing member.
In other words, in an AC based charging method, when charging an image bearing member, the surface potential level of the image bearing member can be made to converge to the predetermined level of Vd uniformly across the entirety of the peripheral surface of the image bearing member by using a DC voltage, as an offset voltage, the potential level of which equals the potential level of the dark area (unexposed portion of charged peripheral surface of image bearing member), whereas when discharging the peripheral surface of the image bearing member, the surface potential level of the image bearing member can be made to converge to 0 V uniformly across the entirety of the peripheral surface of the image bearing member, by using 0 V as the offset voltage. Therefore, an AC based charging method is advantageous over a DC based charging method, a corona based charging device, and the like.
However, in an AC based charging method, leakage or improper charging occurs sometimes since the impedance of a charge roller as a charging member fluctuates due to changes in the charge roller ambience. Thus, it has been a common practice to automatically compensate for the change in the charge roller performance resulting from the changes in the ambience of the charge roller; control was executed so that the current value of the AC voltage applied to the charging roller remained constant. As a commonly used control for keeping constant the amount of electrical current, there are method in which current is kept constant in terms of its peak value, r.m.s. value, and the like.
The aforementioned conventional AC based contact type charging method, however, had the following problems.
In an AC based charging method, positive and negative voltages are alternately applied to a charging member, reversing the direction in which electrical discharge occurs. As a result, the peripheral surface of the photosensitive drum as an object to be charged is substantially deteriorated by this repetitive electrical discharge, and the deteriorated portions of the peripheral surface of the photosensitive drum are shaved away due to the friction caused by such a member as a cleaning blade as a cleaning member which comes into contact with the peripheral surface of the photosensitive drum (photosensitive drum shaving). Therefore, the photosensitive layer of the photosensitive drum becomes gradually thinner, and eventually, it becomes too thin to be effective as a photosensitive layer. For example, it becomes nonuniformly charged in microscopic terms, and/or its charge retaining performance declines. In other words, it fails to be properly charged.
Thus, the length of the service life of an image forming apparatus, or the length of a process cartridge comprising at least a photosensitive drum and a charging member (charge roller), is determined by the number of prints (image formation count) which can be produced before the thickness of the photosensitive layer of the photosensitive drum reduces to its limit.
In recent years, due to environmental problems, increase in printer load (image forming apparatus load) resulting from computer networking, and the like causes, demand for a more durable process cartridge has been increasing; in other words, it has become necessary to increase the length of the service life of a photosensitive drum. As a method for lengthening the service life of an electrophotographic image forming apparatus, there are a few that can be considered, for example, a method in which the initial thickness of the photosensitive layer of a photosensitive drum is increased, and a method in which the amount of the deterioration of a photosensitive drum is reduced by reducing the amount of the electrical discharge from the charge roller as a charging member.
However, simply increasing the thickness of the photosensitive layer of a photosensitive drum degrades the ability of the photosensitive member in terms of surface charge retention, resulting sometimes in a blurred electrostatic image. It has been known that if the amount of the electrical discharge is excessively reduced, the electrical discharge tends to become unstable, and also that if the electrical discharge becomes unstable, the peripheral surface of the photosensitive drum is improperly charged, for example, nonuniformly charged, which results in an unsatisfactory image.
As a method for controlling the AC voltage applied to a charge roller, the aforementioned constant current control is widely used. However, in the constant current control, discharge current (current applied to contact type charging member from power source) which affects the length of the service life of a photosensitive drum is not directly controlled, although the amount of the current which flows to the photosensitive drum from the contact type charging member such as a charge roller is kept constant.
For example, when the amount of the discharge current is kept at an approximate value at which improper charging does not occur in the initial period of the charge roller usage, it is impossible for the efficiency with which a photosensitive drum is charged by a charger roller to be maintained, throughout the entire service life of the photosensitive drum, at the same level as that when both components are brand-new. This is because the manner in which a photosensitive drum is charged changes due to the contamination of a charge roller by toner, and the decrease in the thickness of the photosensitive layer of the photosensitive drum. Thus, in the conventional constant current control in which the AC current level is set so that a photosensitive drum is properly charged in the initial stage, the amount of electrical discharge increases compared to that in the initial stage, increasing thereby the amount of the photosensitive drum shaving as the usage of the photosensitive drum accumulates, in other words, shortening the service life of the photosensitive drum.
The primary object of the present invention is to provide an image forming apparatus capable of preventing its image bearing member from being deteriorated by discharge current while preventing the improper charging of the image bearing member.
Another object of the present invention is to provide an image forming apparatus comprising: an image bearing member; a charging member for charging said image bearing member by making contact with said image bearing member; a voltage applying means for applying oscillating voltage comprising AC voltage to said charging member; a first detecting means for detecting the value of the AC current applied to said charging member; a second detecting means for detecting the value of the said AC current when said AC voltage is at or near its peak; and a controlling means which controls said voltage applying means in such a manner that when the current value detected by said first detecting means is within a predetermined range, the current value detected by said second detecting means remains at a predetermined value, whereas when the current value detected by said first detecting means is outside said predetermined range, the current value detected by said first detecting means remains at a predetermined value.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.